1. Field of the Invention
This invention relates to liquid ink compositions comprising a charge director, a plastisol having either acid or amine groups, and a dispersant. In particular, this invention relates to a liquid ink which exhibits improved dispersion stability and improved chargeability when used in any imaging process, including but not limited to ink transfer processes, ionographic, electrographic and electrophotographic color printing or proofing processes.
2. Background of the Art
Liquid inks are widely used in a variety of imaging and printing processes, for example offset, bubble jet, ink jet, intaglio, rotogravure, electrographic, and electrophotographic printing. Many characteristics desired in pigment dispersions for liquid inks are the same for each of the respective processes even though the final ink formulations may be substantially different. For example, the stability of the pigment dispersion on the shelf, under shear conditions, and under high voltage fields is an important consideration regardless of the final use of the liquid ink. The art continuously searches for more stable pigment dispersions to provide more flexibility in ink formulations to provide better efficiency and waste reduction in the various printing processes.
In electrophotographic applications, which includes devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the photoconductor to radiation in an imagewise pattern, developing a temporary image on the photoconductor by contacting the photoconductor with a liquid ink, and finally transferring the temporary image to a receptor. The final transfer step may be performed either directly from the photoconductor or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
Liquid inks typically comprise an electrically insulating liquid that serves as a carrier for a dispersion of charged particles, known as toner particles. These toner particles are composed of at least a colorant (e.g., pigment or dye) and a polymeric binder. A charge control agent is often included as a component of the liquid developer to regulate the polarity and magnitude of the charge on the toner particles. Liquid inks can be categorized into two primary classes. For convenience, the two classes will be referred to as plastisol inks and organosol inks.
Organsol inks comprise a grafted copolymer comprising a thermoplastic resinous core and a copolymeric steric stabilizer, whereas plastisol inks comprise a plastisol which is a dispersion prepared by dispersing finely divided polymer particles in a liquid medium in which the polymer is insoluble.
One problem in formulating liquid inks is the difficulty in obtaining liquid inks that have excellent dispersion stability. Hence, there is a need for liquid ink compositions having excellent dispersion stability.
An important consideration in formulating liquid inks is the chargeability of the liquid ink. It is desirable for a liquid ink to have a high chargeability so that it can acquire enough quantity of charge to be forced under an electrical field to migrate and then to plate upon the imaged areas on the photoreceptor. The chargeability of a liquid ink is measured by its conductivity and its mobility. In general, liquid inks with proper ink conductivity and proper ink mobility are desirable.
U.S. Pat. No. 4,623,558 (Lin), the disclosure of which is totally incorporated herein by reference, discloses a thermosetting plastisol dispersion composition comprising (1) poly(phenylene oxide) in powder form, which is insoluble in the reactive plasticizer at room temperature and plasticizable at a temperature at or above the fluxing temperature; (2) a liquid reactive plasticizer member of the group consisting of (a) at least one epoxide resin having an average of more than one epoxide group in the molecule, (b) at least one liquid monomer, oligomer, or prepolymer containing at least one ethylenically unsaturated group, and (c) a mixture of (a) and (b), said reactive plasticizer being capable of solvating the poly(phenylene oxide) at the fluxing temperature and being present in an amount ranging from 5 to 2,000 parts per 100 parts by weight of (1); and (3) 0.01 to 10 percent by weight of (2) of either a thermal initiator or photoinitiator for plasticizers present in the composition. The plastisol dispersion after fluxing can form a thermoset after the crosslinking reaction.
This invention provides a stable liquid ink comprising a plastisol with a novel composition that exhibits increased dispersion stability and improved chargeability.
This invention relates to a liquid ink for developing electrophotographic images comprising: a) a carrier liquid having a Kauri-Butanol number less than 30; b) a plastisol; c) a dispersant; and d) a charge director, wherein: for a positive charge director, the dispersant has amine groups and the plastisol comprises polymeric particles having acid groups and an acid value between 30 and 150 mg KOH/g such that the mole ratio of the amine groups to the acid groups is between 0.075 and 0.3; and for a negative charge director, the plastisol comprises polymer particles having an amine value between 5 and 200 mg KOH/g and the dispersant has acid groups such that the mole ratio of the acid groups to the amine groups is between 0.075 and 0.3.
In another aspect, the invention features a plastisol liquid ink that includes:
a) a carrier liquid having a Kauri-Butanol number less than 30;
b) a plastisol wherein a plastic particle component of the plastisol has acid groups and an acid value between 30 and 150 mg KOH/g;
c) a dispersant having amine groups such that the mole ratio of the amine groups to the acid groups is between 0.075 and 0.3; and
d) a positive charge director.
In still another aspect, the invention describes a plastisol liquid ink that includes:
a) a carrier liquid having a Kauri-Butanol number less than 30;
b) a plastisol wherein a plastic particle component of the plastisol has an amine value between 5 and 200 mg KOH/g;
c) a dispersant having acid groups such that the mole ratio of the acid groups to the amine groups is between 0.075 and 0.3; and
d) a negative charge director.
The plastisol liquid inks of the present invention will be described primarily with respect to electrophotographic office printing; however, it is to be understood that these liquid toners are not so limited in their utility and may also be employed in other imaging processes, other printing processes, or other ink transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printer, instrument recording devices, and the like.